A floating inflated structure of cylindrical or similar shape does not have an inherent stability. It requires some sort of ballast in order to be able to take and hold its position, either in a static situation or even in a dynamic situation, during its movement over the water.
In certain conditions of use outside the water such as during periods of transport, for example, the ballast can constitute an inconvenience owing to its bulk and weight.
This problem can equally be found again on classic motor boats such as that described in the document FR 2 633 581. In this document the motor boat comprises a longitudinal keel which is mounted beneath and in the centre of the hull, the said keel being provided in the form of a cylindrical pocket formed with a deformable watertight flexible material which comprises at one of its ends an orifice acting as a vent, and at its other end a fitment in the form of a scoop in order to allow automatic refilling of this pocket with water and this as soon as the boat starts to move.